1. Field of the Invention
The present invention relates to an optical transceiver, and more particularly to a receiver circuit for an optical transceiver having a plurality of photodiodes.
2. Description of the Related Art
An optical receiver converts optical pulses to corresponding electrical signals. The optical receiver in an infrared (IR) optical transceiver typically utilizes an IR-sensitive PIN diode, which emits an electric current in proportion to the intensity of infrared light striking the active area of the diode.
For example, FIG. 1 illustrates the general structure of a conventional optical receiver circuit for an IR transceiver. A PIN diode D has its anode connected to the power Supply Vdd to create a bias voltage across the PIN diode D, and its cathode connected to the inverting input of an operational amplifier AMP. A feedback resistor Rf is connected between the output and the inverting input, so that the operational amplifier AMP operates as a transimpedance amplifier, in that its output voltage Vo is proportional to the input current Ip from the PIN diode:
Vo=xe2x88x92Ip*Rfxe2x80x83xe2x80x83(1)
Ip=Pop*A*RExe2x80x83xe2x80x83(2)
where Pop is the incident optical power intensity, A is the active area of the PIN diode, and RE is the coefficient of responsiveness of the PIN diode.
In equation (1) a negative sign is added to Ip, because if Ip flows from Vdd (positive) to the inverting input of the operational amplifier AMP, the output voltage Vo will be negative. Substituting equation (2) into equation (1) yields:
Vo=xe2x88x92Pop*A*RE*Rfxe2x80x83xe2x80x83(3)
The coefficient of responsiveness RE is determined by the active material used in the PIN diode. The wavelength to which the PIN diode is sensitive will dictate the material used in the diode. Therefore, once the wavelength is determined, RE is fixed and can be considered as a constant.
The design of any conventional IR receiver suffers from PIN diode parasitic capacitance when both high sensitivity and wide bandwidth are required. Generally, the bandwidth of the transimpedance amplifier itself can be very wide, but the total bandwidth of the front edge stage is limited by the single pole RC low pass filter formed by the input capacitance Cin of the operational amplifier AMP and the resistance of the feedback resistor Rf. A single pole RC low pass filter has the following gain-frequency relation:                               G          ⁡                      (            f            )                          =                              G            ⁡                          (              0              )                                            1            +                          j              *              2              *              π              *              f              *              R              *                              C                1                                                                        (        4        )            
where G(f) is the gain as a function of frequency, G(0) is the DC gain, f is the frequency, R is the resistance, C is the capacitance, and j is the imaginary unit.
The bandwidth BW of a single pole RC low pass filter is:                     BW        =                  1                      2            *            π            *            R            *            C                                              (        5        )            
Applying equation (5), the bandwidth of the transimpedance amplifier described above can be expressed as:                               BW          t                =                  1                      2            *            π            *                          R              in                        *                          C              in                                                          (        6        )            
where BWt is the transimpedance amplifier bandwidth, Cin is the capacitance on the transimpedance amplifier inverting input, Rin is the input resistance of the transimpedance amplifier, Rin=Rf/Gopen, and Gopen is the open loop gain of the transimpedance amplifier.
The input capacitance Cin includes PIN diode parasitic capacitance Cp and the operational amplifier input capacitance Ce. Based on current VLSI technology, Ce can be much smaller than Cp (i.e., Cp greater than  greater than Ce), so Ce can be ignored and effectively
Cin=Cpxe2x80x83xe2x80x83(7)
The parasitic capacitance Cp of a PIN diode is proportional to the PIN diode active area:
Cp=K*Axe2x80x83xe2x80x83(8)
where K is a coefficient, and A is the PIN active area.
Substituting equations (7) and (8) into equation (6) yields:
xe2x80x83BWt=1/(2*xcfx80*Rin*K*A)xe2x80x83xe2x80x83(9)
When a high sensitivity is required, a large active area A is needed to get more optical power. However, because the bandwidth BWt is inversely proportional to the active area A, an increase of the area A results in a corresponding decrease in the bandwidth BWt of the circuit. It can be seen from equations (3) and (9) that it is very difficult to achieve both high sensitivity and a wide bandwidth in a single large area PIN diode.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide an optical receiver circuit which replaces a single PIN diode having a large active area with a plurality of (e.g., preferably equal) smaller PIN diodes, each PIN diode being associated with a dedicated element transimpedance amplifier, the outputs of the element transimpedance amplifiers being connected to a summing amplifier which sums the voltages output from the element transimpedance amplifiers.
The parallel structure of the optical receiver circuit of the present invention provides the same output voltage as a single PIN diode having a large active area comparable to the sum of the active areas of the smaller PIN diodes in the optical receiver circuit of the present invention, and thus can achieve the same high sensitivity as the single PIN diode having a large active area. However, the summing amplifier can be designed to have a much wider bandwidth than that of the element transimpedance amplifiers. Since the total bandwidth of the optical receiver circuit depends on the bandwidth of the summed voltages at the input of the summing amplifier, the total bandwidth of the circuit is the sum of the bandwidths of the individual transimpedance amplifiers, which is much greater than that of a receiver circuit utilizing a single PIN diode with a comparable active area.
In a first aspect, the present invention provides an optical receiver circuit, including a plurality of element transimpedance amplifiers and a summing amplifier, the outputs of the element transimpedance amplifiers being connected through resistance elements to an input of the summing amplifier. When a photodiode connected to an input of each element transimpedance amplifier emits a current signal responsive to an optical signal striking the photodiode, a voltage is produced at the output of each element transimpedance amplifier proportional to an intensity of the optical signal, and the voltages output by the element transimpedance amplifiers are summed by the summing amplifier to produce an output electrical signal.
In another aspect, the present invention further provides a photoelectric transceiver having an optical receiver circuit including a plurality of element transimpedance amplifiers, a summing amplifier, the outputs of the element transimpedance amplifiers being connected through resistance elements to an input of the summing amplifier, and a photodiode connected to an input of each element transimpedance amplifier. When the photodiode connected to the input of each element transimpedance amplifier emits a current signal responsive to an optical signal striking the photodiode, a voltage is produced at the output of each element transimpedance amplifier proportional to an intensity of the optical signal, and the voltages output by the element transimpedance amplifiers are summed by the summing amplifier to produce an output electrical signal.
In a further aspect of the present invention, each transimpedance amplifier includes an operational amplifier having a resistance element connected between an output and an inverting input of the operational amplifier, each photodiode is connected to an inverting input of the respective element transimpedance amplifier and the outputs of the element transimpedance amplifiers are connected to an inverting input of the summing amplifier through an input resistor.
In a further aspect of the present invention, the optical signal includes an infrared signal and the photodiodes include PIN diodes.
The present disclosure relates to subject matter contained in Canadian Patent Application No. 2,311,433 filed Jun. 13, 2000, which is expressly incorporated herein by reference in its entirety.